In situ / operando synchrotron x-ray studies of metal additive manufacturing
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Introduction Powder-bed additive manufacturing (AM) processes join material layer-by-layer to make three-dimensional parts from computer models. AM exhibits many advantages over subtractive and formative manufacturing technologies, such as increased part complexity, high customization, short supply chain, on-site and on-demand production, reduction of material and energy consumption. Since AM largely unleashes the freedom to design, topologically optimized parts can now be built with superior performance that were previously inaccessible.1,2 In both the public and private sectors, metal AM has evolved from a rapid prototyping tool to a full-scale product manufacturing technology in the past decade, and has found many applications in aerospace, automobile, medical, defense, and energy industries. Meanwhile, metallurgists have been tackling the tremendous challenges in metal AM and seizing opportunities for fundamental research for designing and processing new alloys and metallic architectures.3–6 Owing to the rapid melting and solidification characteristics of AM, the printed metals exhibit unique microstructures, such as cellular structures, high dislocation densities, solute trapping, unusual chemical segregation, porosity, and nonequilibrium phases.7–14 Some of these structural attributes are favorable for certain applications, while others are deleterious. Understanding the process–structure–property relationship in AM metals is key to avoiding detrimental defect structures
and improving the reliability of printed parts. In an earlier issue of MRS Bulletin, five articles reviewed the unique microstructures (characterized primarily using ex situ techniques) and properties of different alloys processed using AM techniques.15 In this article, we introduce remaining materials issues in metal AM and highlight the recent development of in situ/operando synchrotron x-ray experiments for probing the dynamic energy-matter interaction involved in powder-bed AM and the microstructure evolution during the rapid cooling process. The article ends with a discussion on future research opportunities.
Key processes in laser powder-bed fusion Laser powder-bed fusion (LPBF) is currently the dominant metal AM technique. In a typical LPBF process, as schematically shown in Figure 1, a laser selectively illuminates certain locations on the powder bed to melt the powder and underlying substrate and create a melt pool. Under most build conditions, the laser power is high enough to induce metal evaporation. The metal vapor leaves the melt pool surface with a high speed and creates a recoil pressure that pushes the liquid metal away to form a depression zone. The high-speed vapor jet may also impact on other regions of the depression zone to modify the local fluid flow and interface shape. The general shape of the depression zone varies with the laser parameters (i.e., power and scanning speed), and it often fluctuates with various
Tao Sun, Department of Materials Science and Engineering, University of Virginia, USA; [email protected] W
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